CHARLESTON, W.Va. — U.S. Sen. Robert C. Byrd obtained secret FBI documents about the civil rights movement that were leaked by the CIA and triggered an angry confrontation between the two agencies in the 1960s, according to newly released FBI records.

Byrd, who died in June 2010 at age 92, had sought the FBI intelligence while suspecting that communists and subversives were guiding the civil rights cause, the records show. Decades before he became history's longest-serving member of Congress, or gained the title "King of Pork" for sending federal funds to West Virginia, the Democrat had stalled and voted against major civil rights legislation in the mid-1960s. He also belonged to the Ku Klux Klan while a young man in the 1940s, and the FBI cited that membership while weighing his requests for classified information, the records show.

"He eventually had a change of heart about a lot of that stuff," said Ray Smock, a former historian for Congress who now oversees Byrd's archives. Smock said Byrd's hardline belief in law and order played a role in his view of the civil rights movement. Byrd also repeatedly called his time with the hate group a serious mistake, Smock noted.

The FBI released more than 750 pages from its files — many of them with words, sentences or entire paragraphs redacted — in response to a Freedom of Information Act request by The Associated Press. The records date to the mid-1950s, when Byrd served in the U.S. House. He was elected to the first of his record nine terms in the U.S. Senate in 1958.

The documents that reveal the September 1966 leak also describe how it sparked outrage among top FBI officials and prompted an internal CIA probe that singled out two agency employees as the culprits The episode damaged Byrd's standing with the bureau, though only briefly, the records show. Numerous documents depict him as an outspoken supporter of the FBI and particularly of J. Edgar Hoover, its longtime director, even toward the end of Hoover's tenure as criticism of him mounted.

"Byrd said that the Director's record of public service was unparalleled anywhere and he knew that it would never be possible for any successor to adequately 'fill his shoes,'" one June 1966 memo between top FBI brass said.

The files repeatedly refer to Byrd's "cordial relations" with the bureau, and include numerous thank-you notes and other friendly exchanges between Byrd and Hoover from the early 1960s until Hoover's death in 1972.

"He certainly was a law and order conservative," said Smock, director of the Robert C. Byrd Center for Legislative Studies at Shepherd University in West Virginia. "He had great respect for the Justice Department and for Hoover, as far as I know."

The FBI had provided Byrd only with publicly available information about three unidentified individuals involved in civil rights matters when he revealed the leaked documents to an FBI agent during a September 1966 meeting, a memo to FBI Deputy Director C.D. DeLoach said.

"Why can't a United State Senator, the best friend the FBI has in the Senate, get information directly from the FBI which he has already received from a third party," Byrd was quoted as saying. The memo said Byrd then showed the agent Xerox copies of two secret FBI investigative reports and one internal memo.

Byrd refused to reveal his source, but markings on the documents led the FBI to conclude they were copies of papers provided to the CIA earlier that year.

"(I)t is believed that the Director of CIA should be fully aware of this situation and if the CIA is guilty, as it appears to be, (Director Richard) Helms should be emphatically impressed with our displeasure for such uncalled-for activity," the memo said.

Several of these typed memos feature handwritten notes underscoring the bureau's anger and concern, some signed with Hoover's initial. "This is outrageous breach of security by CIA," one such note read. Helms, meanwhile, was "decidedly disturbed" when told of the leak, another memo said.

"Helms replied that it was difficult for him to believe that anyone would be so stupid to become involved in such activity, but he has learned through bitter experience that 'anything is possible,'" that document said.

The records show that Helms' security chief, Howard Osborne, ran down the leak: Two CIA employees provided the documents to an unidentified Maryland county law enforcement official, who then handed them off to the senator.

Osborne told the FBI that both "he and Mr. Helms are distressed over the incident and that they intend to make an example of the guilty CIA employees to insure that such an incident never occurs again," a follow-up memo to DeLoach said.

The employees' final fate is unclear, though the memo said Osborne was confident Helms would support firing one or both of the culprits.